Rosin and alum sizing of paper and paperboard has been well established in the art for many years, and is widely employed to produce water repellent paper products. The production of such papers uses rosin size, which is obtained from "tall oil," a naturally occurring product extracted from soft wood trees such as pines. This oil is saponified by the addition of caustic soda to produce the sodium salt of the oil, or rosin soap. Rosin soap is also produced as a byproduct of the alkaline pulping of soft woods, and is found in various concentrations in unbleached kraft soft wood pulp. This material is used extensively in food wraps and other papers and paperboard in which water repellence is desirable.
Although the pulp, or "stock" as it is called in the art, contains a certain concentration of rosin soap, it is common practice to disperse additional rosin soap in the stock, to further increase the water repellent characteristics of the paper to be produced. The rosin soap may be modified chemically to increase its ability to repel water, to promote dispersion into stock, to put the rosin in oil form before it is used, and to produce aqueous dispersions having high rosin content to facilitate shipping at low costs.
In order to retain, or set, the rosin in the paper during the manufacturing process, when water is removed from the stock, a retention agent is added to the stock. Such retention agents commonly include polymers and, more commonly, alum (Al.sub.2 (SO.sub.4).sub.3), which is typically provided in an aqueous solution 27 percent by weight anhydrous Al.sub.2 (SO.sub.4).sub.3. When added to sufficient quantities of water, alum hydrolyzes to form aluminum hydroxide and sulfuric acid. The sulfuric acid thus formed lowers the stock pH. As the stock pH becomes acid, the rosin soap is converted to rosin oil, which is also commonly known as rosin size. The aluminum hydroxide retains the rosin size.
Rosin size may be chemically modified, as noted above to enhance certain characteristics such as water repellency. In addition, there are artificial sizes, such as AKD (alkyl ketene dimer) or ASA (alkynyl succinic anhydride), that are substitutes for rosin size.
At the same time, the aluminum hydroxide thus produced acts to flocculate rosin size onto the paper fibers in the wet stock. This compound also acts as a retention agent for anionic substances such as rosin size, retaining the size on the paper fibers even as water is removed from the stock; it most efficaciously serves this purpose at a pH range of 4.0 to 5.5. In order to lower the stock pH sufficiently to cause the aluminum hydroxide to act as a retention agent, additional sulfuric acid is commonly added to the stock. It has been shown that this process typically requires two pounds of alum (Al.sub.2 SO.sub.4).sub.3 .multidot.18H.sub.2 O) to set one dry pound of rosin size.
The rosin size thus retained in the dry paper is an oil that causes the finished paper to repel water.
The aluminum hydroxide also provides other functions resulting from flocculation of not only paper fibers but also fines. Notably, increased flocculation results in better drainage of water from the stock (measured as "freeness"), resulting in faster and easier drying of the stock into paper.
Another function of the addition of alum is that gums, fillers, starches, dry strength additives and other additives used to impart desirable properties in the finished paper are retained with the fibers, reducing the presence of such materials in the process water. In addition, as a result of retention of the materials in the paper, these additives may be used more effectively and economically.
All of this results in lower concentrations of pollutants in effluent waste streams, lower head box consistency, and lower head box freeness. This also results in less loss of stock and facilitates solid waste removal using clarifiers, savealls, screens or filters.
Unfortunately, alum is an expensive material, costing up to hundreds of dollars per ton of material. This represents an increase in the cost of materials employed in the production of water repellent papers, adding significantly to the cost of each ton of paper produced with alum.
Another problem is that alum also operates efficaciously as a flocculant only in a narrow, acidic pH range from 4.0 to 5.5. In this range, the acidic stock corrodes the extremely expensive paper making equipment, requiring repair or replacement of the equipment and shutting down production while repair or replacement is occurring. Alternatively, expensive acid resistant materials are used in such equipment, adding to their cost.
A problem also results from the deposition of aluminum hydroxide on the paper making equipment as the stock is converted to paper, frequently shutting down the equipment for cleaning. Expensive chemicals specially prepared for removal of aluminum hydroxide are required to clean the equipment and return it to service.
Moreover, alum is a very hygroscopic material, and as such detrimentally increases the time and cost to dry the paper.
For these reasons, it has long been felt by those in the paper industry that a cost-effective substitute for alum as a retention aid for rosin size was desirable.
Recently, it is also believed by some medical experts that aluminum and aluminum salts play a role in Alzheimer's disease, a serious affliction impairing the physical and mental abilities of thousands of persons each year. Since many of the products in which alum is employed as a retention agent are used in food packaging, a potential health threat resulting from the use of this material may exist. Alum is also present in the atmosphere surrounding paper production equipment, thereby posing a health risk to workers using such equipment.